text: /*
   * Use DWARF Debug information to skip unnecessary callchain entries.
   *
   * Copyright (C) 2014 Sukadev Bhattiprolu, IBM Corporation.
   * Copyright (C) 2014 Ulrich Weigand, IBM Corporation.
   *
   * This program is free software; you can redistribute it and/or
   * modify it under the terms of the GNU General Public License
   * as published by the Free Software Foundation; either version
   * 2 of the License, or (at your option) any later version.
   */
  #include <inttypes.h>
  #include <dwarf.h>
  #include <elfutils/libdwfl.h>
  
  #include "util/thread.h"
  #include "util/callchain.h"
  #include "util/debug.h"
  
  /*
   * When saving the callchain on Power, the kernel conservatively saves
   * excess entries in the callchain. A few of these entries are needed
   * in some cases but not others. If the unnecessary entries are not
   * ignored, we end up with duplicate arcs in the call-graphs. Use
   * DWARF debug information to skip over any unnecessary callchain
   * entries.
   *
   * See function header for arch_adjust_callchain() below for more details.
   *
   * The libdwfl code in this file is based on code from elfutils
   * (libdwfl/argp-std.c, libdwfl/tests/addrcfi.c, etc).
   */
  static char *debuginfo_path;
  
  static const Dwfl_Callbacks offline_callbacks = {
  	.debuginfo_path = &debuginfo_path,
  	.find_debuginfo = dwfl_standard_find_debuginfo,
  	.section_address = dwfl_offline_section_address,
  };
  
  
  /*
   * Use the DWARF expression for the Call-frame-address and determine
   * if return address is in LR and if a new frame was allocated.
   */
  static int check_return_reg(int ra_regno, Dwarf_Frame *frame)
  {
  	Dwarf_Op ops_mem[2];
  	Dwarf_Op dummy;
  	Dwarf_Op *ops = &dummy;
  	size_t nops;
  	int result;
  
  	result = dwarf_frame_register(frame, ra_regno, ops_mem, &ops, &nops);
  	if (result < 0) {
  		pr_debug("dwarf_frame_register() %s\n", dwarf_errmsg(-1));
  		return -1;
  	}
  
  	/*
  	 * Check if return address is on the stack.
  	 */
  	if (nops != 0 || ops != NULL)
  		return 0;
  
  	/*
  	 * Return address is in LR. Check if a frame was allocated
  	 * but not-yet used.
  	 */
  	result = dwarf_frame_cfa(frame, &ops, &nops);
  	if (result < 0) {
  		pr_debug("dwarf_frame_cfa() returns %d, %s\n", result,
  					dwarf_errmsg(-1));
  		return -1;
  	}
  
  	/*
  	 * If call frame address is in r1, no new frame was allocated.
  	 */
  	if (nops == 1 && ops[0].atom == DW_OP_bregx && ops[0].number == 1 &&
  				ops[0].number2 == 0)
  		return 1;
  
  	/*
  	 * A new frame was allocated but has not yet been used.
  	 */
  	return 2;
  }
  
  /*
   * Get the DWARF frame from the .eh_frame section.
   */
  static Dwarf_Frame *get_eh_frame(Dwfl_Module *mod, Dwarf_Addr pc)
  {
  	int		result;
  	Dwarf_Addr	bias;
  	Dwarf_CFI	*cfi;
  	Dwarf_Frame	*frame;
  
  	cfi = dwfl_module_eh_cfi(mod, &bias);
  	if (!cfi) {
  		pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1));
  		return NULL;
  	}
  
  	result = dwarf_cfi_addrframe(cfi, pc-bias, &frame);
  	if (result) {
  		pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1));
  		return NULL;
  	}
  
  	return frame;
  }
  
  /*
   * Get the DWARF frame from the .debug_frame section.
   */
  static Dwarf_Frame *get_dwarf_frame(Dwfl_Module *mod, Dwarf_Addr pc)
  {
  	Dwarf_CFI       *cfi;
  	Dwarf_Addr      bias;
  	Dwarf_Frame     *frame;
  	int             result;
  
  	cfi = dwfl_module_dwarf_cfi(mod, &bias);
  	if (!cfi) {
  		pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1));
  		return NULL;
  	}
  
  	result = dwarf_cfi_addrframe(cfi, pc-bias, &frame);
  	if (result) {
  		pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1));
  		return NULL;
  	}
  
  	return frame;
  }
  
  /*
   * Return:
   *	0 if return address for the program counter @pc is on stack
   *	1 if return address is in LR and no new stack frame was allocated
   *	2 if return address is in LR and a new frame was allocated (but not
   *		yet used)
   *	-1 in case of errors
   */
  static int check_return_addr(struct dso *dso, u64 map_start, Dwarf_Addr pc)
  {
  	int		rc = -1;
  	Dwfl		*dwfl;
  	Dwfl_Module	*mod;
  	Dwarf_Frame	*frame;
  	int		ra_regno;
  	Dwarf_Addr	start = pc;
  	Dwarf_Addr	end = pc;
  	bool		signalp;
  	const char	*exec_file = dso->long_name;
  
  	dwfl = dso->dwfl;
  
  	if (!dwfl) {
  		dwfl = dwfl_begin(&offline_callbacks);
  		if (!dwfl) {
  			pr_debug("dwfl_begin() failed: %s\n", dwarf_errmsg(-1));
  			return -1;
  		}
  
  		mod = dwfl_report_elf(dwfl, exec_file, exec_file, -1,
  						map_start, false);
  		if (!mod) {
  			pr_debug("dwfl_report_elf() failed %s\n",
  						dwarf_errmsg(-1));
  			/*
  			 * We normally cache the DWARF debug info and never
  			 * call dwfl_end(). But to prevent fd leak, free in
  			 * case of error.
  			 */
  			dwfl_end(dwfl);
  			goto out;
  		}
  		dso->dwfl = dwfl;
  	}
  
  	mod = dwfl_addrmodule(dwfl, pc);
  	if (!mod) {
  		pr_debug("dwfl_addrmodule() failed, %s\n", dwarf_errmsg(-1));
  		goto out;
  	}
  
  	/*
  	 * To work with split debug info files (eg: glibc), check both
  	 * .eh_frame and .debug_frame sections of the ELF header.
  	 */
  	frame = get_eh_frame(mod, pc);
  	if (!frame) {
  		frame = get_dwarf_frame(mod, pc);
  		if (!frame)
  			goto out;
  	}
  
  	ra_regno = dwarf_frame_info(frame, &start, &end, &signalp);
  	if (ra_regno < 0) {
  		pr_debug("Return address register unavailable: %s\n",
  				dwarf_errmsg(-1));
  		goto out;
  	}
  
  	rc = check_return_reg(ra_regno, frame);
  
  out:
  	return rc;
  }
  
  /*
   * The callchain saved by the kernel always includes the link register (LR).
   *
   *	0:	PERF_CONTEXT_USER
   *	1:	Program counter (Next instruction pointer)
   *	2:	LR value
   *	3:	Caller's caller
   *	4:	...
   *
   * The value in LR is only needed when it holds a return address. If the
   * return address is on the stack, we should ignore the LR value.
   *
   * Further, when the return address is in the LR, if a new frame was just
   * allocated but the LR was not saved into it, then the LR contains the
   * caller, slot 4: contains the caller's caller and the contents of slot 3:
   * (chain->ips[3]) is undefined and must be ignored.
   *
   * Use DWARF debug information to determine if any entries need to be skipped.
   *
   * Return:
   *	index:	of callchain entry that needs to be ignored (if any)
   *	-1	if no entry needs to be ignored or in case of errors
   */
  int arch_skip_callchain_idx(struct thread *thread, struct ip_callchain *chain)
  {
  	struct addr_location al;
  	struct dso *dso = NULL;
  	int rc;
  	u64 ip;
  	u64 skip_slot = -1;
  
  	if (chain->nr < 3)
  		return skip_slot;
  
  	ip = chain->ips[2];
  
  	thread__find_symbol(thread, PERF_RECORD_MISC_USER, ip, &al);
  
  	if (al.map)
  		dso = al.map->dso;
  
  	if (!dso) {
  		pr_debug("%" PRIx64 " dso is NULL\n", ip);
  		return skip_slot;
  	}
  
  	rc = check_return_addr(dso, al.map->start, ip);
  
  	pr_debug("[DSO %s, sym %s, ip 0x%" PRIx64 "] rc %d\n",
  				dso->long_name, al.sym->name, ip, rc);
  
  	if (rc == 0) {
  		/*
  		 * Return address on stack. Ignore LR value in callchain
  		 */
  		skip_slot = 2;
  	} else if (rc == 2) {
  		/*
  		 * New frame allocated but return address still in LR.
  		 * Ignore the caller's caller entry in callchain.
  		 */
  		skip_slot = 3;
  	}
  	return skip_slot;
  }
  